This invention relates generally to centrifugal pumps, and, more particularly to lubricating pump seals for centrifugal pumps that act to reduce wear between the rotating and stationary surfaces of pumps used to pump a mixture of solids and carrier liquid, commonly known as slurry.
Centrifugal pumps, as the name implies, employ centrifugal force to life liquids from a lower to a higher level or to produce a pressure. This type of pump, in its simplest form, comprises an impeller consisting of a connecting hub with a number of vanes and shrouds, rotating in a volute collector or casing (See FIGS. 1 and 2). Liquid drawn into the center, or eye, of the impeller is picked up by the vanes and accelerated to a high velocity by rotation of the impeller. It is then discharged by centrifugal force into the casing and out the discharge branch of the casing. When liquid is forced away from the center of the impeller, a vacuum is created, causing more liquid to flow into the center of the impeller. Consequently there is a continuous flow through the pump.
The rotation of the impeller vanes results in a higher pressure in the volute collector than in the suction, which results in flow. This higher pressure has to be sealed against the lower pressure suction on one side and where the shaft (at a lower atmospheric pressure) on the other side enters the collector, to avoid leakage losses and loss of performance. In the case of the shaft, the most common sealing method is to utilize a stuffing box with rings of packing. On the front, or suction side, with water pumps the most common method of sealing is to utilize a close radial clearance between the impeller and the casing and to employ radial seal rings. For pumps used to pump slurry, the sealing problem is more difficult. While radial seal rings are effective in clean water pump applications, experience has shown that the particles being pushed through the gap between the sealing surfaces are thrown off the rotating radial surface of the impeller seal ring causing high wear to the wetted surfaces of the pump.
Wear occurs mostly as a result of particles impacting or sliding on the wetted surfaces. The amount of wear depends on the particle size, shape, specific gravity of the solids, and sharpness of the particulate matter, most of which is dictated by the service and the velocity of impact (or concentration) of the solids.
In order to redue wear, some pumps employ a water flush to dilute and exclude particles, some utilize semi-axial gaps tapering inwards at an angle, and some utilize clearing vanes protruding out of the front shroud of the impeller into the gap between the impeller and the suction liner. Each of these, however, has either not satisfactorily solved the problem of wear, or has reduced wear at the expense of pump efficiency. What is needed is a seal construction that is simple, effective in reducing wear, and that does not impair the performance of the pump.
The present invention is directed to a simple, cost-effective sealing assembly for centrifugal pumps that addresses each of the problems described above specifically, the sealing assembly is adaptable for use in a centrifugal pump of the type used for pumping an abrasive slurry where wear due to particulate matter is particularly problematic. The seal assembly may be installed in a pump that has, or can be modified to include, a sealing ring groove in the stationary pump casing and a means for supplying clean, pressurized flush water into the sealing ring groove. While the present invention may be installed on a variety of pump types, exemplary installation on a single-stage, single-suction centrifugal pump will be explained in detail herein.
One embodiment of the present invention includes a radial seal that is positioned within the sealing ring groove of the stationary pump casing of a centrifugal pump. The radial seal is dimensioned to be smaller than the groove so that it may freely move within the groove. The radial (circular) seal has a generally rectangular cross section and is formed of a wear-resistant malleable iron, elastomer, or ceramic material, though an elastomer is preferred.
The seal comprises a sealing, or forward, end and a flushing water inlet, or rear, end. The rear end of the seal includes at least one orifice through which pressurized seal water is received. The sealing end of the seal includes multiple, spaced perforations through which the flush water evenly flows. The perforations are sized so that their combined surface area is less than the area of the inlet surface orifice of the seal or the sealing ring groove. This ensures that when pressurized flush water is supplied to the sealing ring groove, the pressure forces the seal to protrude outwardly from the sealing ring groove into the gap between the stationary pump casing and the rotating impeller surface. Hydrostatically, as the seal approaches the surface of the impeller, back pressure between the impeller and the radial seal increases. This balances the pressure on the seal so that the seal does not directly contact the impeller with any significant rubbing force. In this manner, seal wear is minimized, while the seal water provides lubrication and cleaning of the wetted surfaces.
In another embodiment, the sealing end of the seal of the present invention has a centrally-formed recessed region. Desirably, it creates a xe2x80x9cshower headxe2x80x9d, or conical, distribution of flush water. Formed in this fashion, the flush water is caused to spread out from the perforations onto an even larger predetermined surface area. When the flush water enters the recessed portion, pressure in the recessed portion builds, again balancing the hydrostatic force between the seal and the impeller surface, so that the seal moves outward, but never actually contacts the impeller.
In both of the described embodiments of the present invention, the hydrostatic balance between the seal and the pump impeller forms a xe2x80x9cself-compensatingxe2x80x9d clearance that not only reduces pump wear, but also ensures more efficient pump operation.
These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiments when considered in conjunction with the drawings. It should be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.